1. Field of the Invention
The present invention relates to an inkjet printing method and device and, in particular, to an on-demand inkjet printing method and device for printing characters and/or images for use in a printer, a plotter, a facsimile device, a copying machine or the like.
2. Description of the Related Art
Printing devices such as printers are essential in the recent office automation environment, and even personal use printing devices have been widely spreading. Among them, with respect to the printers attention has been more paid to inkjet printers as compared with wire printers which perform printing by magnetically driving wires to press them onto a platen via an ink ribbon and print medium such as a print sheet of paper. As appreciated as compared with the wire printer, the inkjet printer produces less noise and carries out high speed printing with less printing cost per sheet.
In the inkjet printing, ink drops of different volumes or sizes are injected for forming dots if different sizes on a print medium so as to realize a halftone printing. In this case, the ink drops are jetted successively at constant periods (T|sec|).
Normally, the multi pass printing is carried out wherein ink drops of the same site are successively jetted on one line, then ink drops of another same size are successively jetted on the same line, which are repeated to jet the ink drops of various sizes without changing the line.
In the foregoing halftone printing, however, there has been a serious problem that a disorder or an output image is caused due to the fact that dots are not formed at predetermined positions on the print medium even with the normal injection timings.
Although such a disorder of the output image is prevented in the multi pass printing, there is a drawback that the printing speed is lowered.
There has also been a problem that when drive period of voltages applied to a piezoelectric element is changed, dot dislocation on a print medium occurs to lower the printing quality.
There has also been a problem at in some cases, an extra ink drop is injected to lower the printing quality.
There has also been a problem that as a time for which no ink drop is injected via a nozzle is prolonged, a hit position of on ink drop and a dot size on a print medium can not be correctly controlled.
The present inventors tried to seek reasons why the disorder of the output image is caused and found out one of the reasons that the ink drops hit upon the print medium at positions other than the predetermined positions due to differences in size of the ink drops. Specifically, when the ink drops of different sizes are injected, the fling speed increases as the volume or mass of the ink drop increases. As speed differences among the ink drops increase, the accuracy of the hit positions of the ink drops on the print medium is lowered to degrade the quality of the output image.
Another reason is also found out that since meniscus vibration after an injection of an ink drop can not be suppressed, an extra ink drop (hereinafter referred to as a xe2x80x9csatellite dropxe2x80x9d) is injected to degrade the quality of the output image.
Another reason is also found out that as a time for which no ink drop is injected via a nozzle is prolonged, water contained in ink near a nozzle opening is gradually vaporized so that the mixing ratio of ink components as well as materiality values (viscosity, density, surface tension, etc.) are changed. Thus, for example, when the viscosity is increased, an injection ink amount is reduced to diminish a dot size on the print medium or lower an ink drop speed (average speed while ink flies as an ink drop). As a result, the hit position of the ink drop on the print medium can not be correctly controlled.
Therefore, it is an object of the present invention to provide an improved inkjet printing method that can eliminate one or more of the disadvantages inherent in the prior art.
It is another object of the present invention to provide an improved inkjet printing device that can eliminate one or more of the disadvantages inherent in the prior art.
According to a first aspect of the present invention, there is provided an inkjet printing method wherein an ink drop is injected via a nozzle by changing drive voltages applied to a piezoelectric element to reduce the volume of a pressure chamber filled with ink, so that the injected ink drop hits upon a print medium to carry out printing, the inkjet printing method comprising: changing, upon injecting the ink drop via the nozzle, an injection timing of the ink drop corresponding to a drive frequency of the drive voltages using a predetermined rule which is prestored.
It may be arranged that the prestored predetermined rule comprises a table defined in terms of drive frequencies of the drive voltages and optimum injection timings of the ink drop corresponding to the drive frequencies.
According to a second aspect of the present invention, there is provided an inkjet printing method wherein all ink drop is injected via a nozzle by changing drive voltages applied to a piezoelectric element to reduce the volume of a pressure chamber of an inkjet head filled with ink, so that the injected ink drop hits upon a print, medium to carry out printing, the inkjet printing method comprising: controlling the inkjet head upon injecting the ink drop via the nozzle such that T3xe2x89xa6Tc or (T2xc2x7T3)xe2x89xa6Tc, wherein T2 represents a time for rapidly contracting the pressure chamber after T1 representing a time for expanding the pressure chamber, T3 represents a time for holding the contracted state of the pressure chamber or further contracting the pressure chamber gradually, T4 represents a time for restoring the pressure chamber to an initial state, and Tc represents a period given by 1/Helmholz resonance frequency of the pressure chamber.
According to a third aspect of the present invention, there is provided on inkjet printing method wherein an ink drop is injected via a nozzle by changing drive voltages applied to a piezoelectric element to reduce the volume of a pressure chamber filled with ink, so that the injected ink drip hits upon a print medium to carry out printing, the inkjet printing method comprising: determining a waveform of the drive voltages corresponding to a flying interval of the ink drop using a predetermined rule which is prestored, the flying interval being defined as a time from an injection of an ink drop to an injection of a subsequent ink drop.
It may be arranged that the waveform of the drive voltages includes two continuous rising portions following a descend portion and having different inclinations, and wave heights of the two rising portions are defined in the prestored predetermined rule.
It may be arranged that the prestored predetermined rule comprises a table defined in terms or flying intervals of ink drops to be injected via the nozzle and optimum waveforms of drive voltages corresponding to the flying intervals.
According to a fourth aspect of the present invention, there is provided an inkjet printing device wherein an ink drop is injected via a nozzle by changing drive voltages applied lo a piezoelectric element to reduce the volume of a pressure chamber filled with ink, so that the injected ink drop hits upon a print medium to carry out printing, the inkjet printing device characterized by: changing, upon injecting the ink drop via the nozzle, an injection timing of the ink drop corresponding to a drive frequency of the drive voltages using a predetermined rule which is prestored.
It may be arranged that the prestored predetermined rule comprises a table defined in terms of drive frequencies of the drive voltages and optimum injection timings of the ink drop corresponding to the drive frequencies.
According to a fifth aspect of the present invention, there is provided an inkjet printing device wherein an ink drop is injected via a nozzle by changing drive voltages applied to a piezoelectric element to reduce the volume of a pressure chamber of an inkjet head filled with ink, so that the injected ink drop hits upon a print medium to carry out printing, the inkjet printing device characterized by: controlling the inkjet head upon injecting the ink drop via the nozzle such that T3xe2x89xa6Tc or (T2+T3)xe2x89xa6Tc, wherein T2 represents a time for rapidly contracting the pressure chamber after T1 representing a time for expanding the pressure chamber, T3 represents a time for holding the contracted state of the pressure chamber of further contracting the pressure chamber to an initial state, and Tc represents a period given by 1/Helmholtz resonance frequency of the pressure chamber.
According to a sixth aspect of the present invention, there is provided an inkjet printing device wherein an ink drop is injected via a nozzle by changing drive voltages applied to a piezoelectric element to reduce the volume of a pressure chamber filled with ink, so that the injected ink drop hits upon a print medium to carry out printing, the inkjet printing device characterized by: determining a waveform of the drive voltages corresponding to a flying interval of the ink drop using a predetermined rule which is prestored, the flying interval being defined as a time from an injection of an ink drop to an injection of a subsequent ink drop.
It may be arranged that the waveform of the drive voltages includes two continuous rising portions following a descending portion and having different inclinations, and wave heights of the two rising portions are defined in the prestored predetermined rule.
It may he arranged that the prestored predetermined rule comprises a table defined in terms of flying intervals of ink drops to be injected via The nozzle and optimum waveforms of drive voltages corresponding to the flying intervals.